Recently, interest has been focused on diode lasers as a possible light source for integrated optical circuits. When the PN junction of these devices is forward-biased, charge carriers of one type are injected across the PN junction to a waveguide region which has a predominance of charged carriers of the opposite conductivity type, with recombination of the carriers producing light. Feedback of light necessary to produce lasing is achieved by cleaved end-faces and/or external mirrors with one or both of the external mirrors being about 70% transmissive so that the output light beam can be transmitted.
In conventional diode lasers, of the type described, especially single heterojunction and double heterojunction diode lasers, the light waveguide layer of the device is extremely thin (approximately 1.5 microns) and laser oscillations occur across filamentary areas on the order of only 10 microns wide. As a result, the laser output light beam eminates from virtually a "slit" source at an edge of the waveguide layer and the divergence of the output beam is large since the beam divergence is inversely proportional to the size of the slit aperature. Typical beam divergences are 10.degree. in one direction and possibly 30.degree.-40.degree. in the other direction. Also, the laser output beam obtained via transmission through cleaved end faces often has a random polarization. These two factors, i.e., large divergence angle and randomness of polarization, has stimulated interest in lasers in which the output light beam is emitted normal to, or substantially normal to, the plane of the waveguide layer of a laser as discussed in an article appearing in Applied Physics Letters, Vol. 25, No. 4, Aug. 1974 and entitled "Selectively Etched Diffraction Gratings in GaAs", and in an article appearing in Soviet Physics-Semiconductors, Vol. 6, No. 7, page 1184 (1973) entitled "Injection Heterojunction Laser With A Diffraction Grating On Its Contact Surface".